1. Field of the Invention
This invention related to optical scanning systems and particularily to scanning apparatus having a scanning range including a large depth of field. The invention is especially suitable for use in a bar code scanner for reading the bar codes of the type usually found on consumer products, for example the uniform grocery product or UPC code.
2. Description of the Invention
Various scanning systems have been developed for many processing industries to read bar code symbols, which uniquely identify an object to be processed, to decode the symbol to a multiple digit representation for record keeping and inventory control purposes. For example, blood banks have begun to identify blood bags with bar code symbols to facilitate processing. In the processing and packaging of many drugs, and other industrial products there is a need to rapidly and accurately identify, and differentiate between many products.
The magazine distribution industry widely distributes magazines on a consignment basis, and therefore the need exists for accurately and quickly tallying the returned magazines for credit validation. Similarly, the record and tap industry requires accurate inventory information of its sold and returned products.
There are several scanning systems in current use. For example there are point of sale or deck scanners which are large, stationary installations which are built into an immovable structure such as a super market counter. In these scanners the symbol, or bar code, must be oriented face-down during passage of the object through the system. Other systems include contact-type and non-contact-type wand bar code readers which are positioned near the symbol, and manually dragged across the symbol. In such applications, and particularly in the non-contact type of bar code readers, it is critical for the operator to maintain the bar code reader an exact distance from the symbol, and to aim the bar code reader in a uniform manner in order to maintain a uniform depth of field, or focal point to properly read the bar code.
Other bar code scanners, which are generally available, use a source of coherent light from a laser and scan the light in various directions across a window on which the object having the code is placed. Because the laser illumination is intense and the aperture stop is small, or F number (F#) is large, the scanner is not sensitive so far as depth of field is concerned and the bar code may be located over a large distance of several inches forwardly of the window. The bar code signal is obtained by a photodetector which is responsive to light scattered from the bar code as the laser beam scans across the code. An example of a laser bar code scanner may be had by reference to U.S. Pat. No. 3,978,317, issued Aug. 31, 1976. The use of lasers in bar code scanners is disadvantageous because of their expense and also because intense laser light is regarded as a health hazard. As a result laser devices must be registered with the U.S. Bureau of Radiological Health in the United States.
Accordingly, it is desirable to utilize the non-coherent light of conventional lamp or light-emitting diodes, in optical scanners. The illumination of the code with non-coherent light is not as intense as the laser light. Moreover, in order to be practical for commercial use the scanner must be able to read codes located over a wide depth of field in front of the scanner as well as be tolerant of tilt and skew within the depth of the field. As the bar codes become smaller high resolution capability is essential on the code reader. When a small aperture stop is used in the optical system of the reader corresponding to a large F/# in order to get a large depth of field, the system is limited by noise when non-coherent light or white light is used. The noise characteristics are constant and depend on upon the noise generated in the photodetection process to a large extent. Thus when the F/# is high, the intensity of the illumination on the photodetector is reduced. The system is then signal to noise limited and incapable of resolving fine or high density bar codes. The problem is still further exacerbated when it is desired to use a non-coherent light in a hand held scanner.
It is a principal object of the present invention to provide an improved system for optically scanning over a large depth of field.
It is a further object of the present invention to provide an improved optical scanner capable of reading bar codes with high resolution even though non-coherent or white light is used as the source of illumination of the code.
It is still a further object of the present invention to provide an improved scanner for bar codes and the like which uses a non-coherent light source, and which is suitable for reading fine or high density bar codes.